There are ways to convert dictionaries in other popular formats for use with OSX Dictionary.app.
For example, to convert a Babylon dictionary to AppleDict, all it takes is:
pyglossary.pyw --read-options=resPath=OtherResources --write-format=AppleDict webster.bgl webster.xml
The full list of formats supported by pyglossary: https://github.com/ilius/pyglossary/blob/master/README.rst - move up several dirs up to get the git clone URL.
I had to fiddle for a while with reading readme's and installing the additional Apple Dev Extensions but it worked for me in the long run.
Considering that there are hundreds of good dictionaries in ABBYY Lingvo DSL format, I have also made a script that automates the conversion process for DSL: https://gist.github.com/elFua/8540294
Again, all of this will not work out of the box, and you'll first need to make sure all the prerequisites are satisfied.